Editorials

PEEP, Auto-PEEP, and Waterfalls

To assess the effect of the prone position on physiological measures, including inspiratory effort, metabolic cost of breathing, and neural drive to the diaphragm as compared with the supine position in infants with severe bronchiolitis requiring noninvasive ventilation.

Fourteen infants, median age 33 days (IQR [first and third quartiles], 25-58) were randomized to receive 7 cmH₂O continuous positive airway pressure for 1 hour in the prone position or in the supine position, which was followed by cross-over to the supine position and the prone position for 1 hour, respectively. Flow, esophageal, airway, gastric, and transdiaphragmatic pressures, as well as electrical activity of the diaphragm were simultaneously recorded. The modified Wood clinical asthma score was also assessed.

Median esophageal pressure–time product per minute was significantly lower in the prone position than in the supine position (227 cmH₂O*s/minute [IQR, 156-282] cmH₂O*s/minute vs 353 cmH₂O*s/minute [IQR, 249-386 cmH₂O*s/minute]; P = .048), as were the modified Wood clinical asthma score (P = .033) and electrical activity of the diaphragm (P = .006). The neuromechanical efficiency of the diaphragm, as assessed by transdiaphramagtic pressure to electrical activity of the diaphragm swing ratio, was significantly higher in the prone position than in the supine position (1.1 cmH₂O/µV [IQR, 0.9-1.3 cmH₂O/µV] vs 0.7 cmH₂O/µV [IQR, 0.6-1.2 cmH₂O/µV], respectively; P = .022).

This study suggests a benefit of the prone position for infants with severe bronchiolitis requiring noninvasive ventilation by significantly decreasing the inspiratory effort and the metabolic cost of breathing. Further studies are needed to evaluate the potential impact of these physiological findings in a larger population.

Clinicaltrials.gov: NCT02602678.

Expiratory flow limitation (EFL) is present when the flow cannot rise despite an increase in the expiratory driving pressure. The mechanisms of EFL are debated but are believed to be related to the collapsibility of small airways. In patients who are mechanically ventilated, EFL can exist during tidal ventilation, representing an extreme situation in which lung volume cannot decrease, regardless of the expiratory driving forces. It is a key factor for the generation of auto- or intrinsic positive end-expiratory pressure (PEEP) and requires specific management such as positioning and adjustment of external PEEP. EFL can be responsible for causing dyspnea and patient-ventilator dyssynchrony, and it is influenced by the fluid status of the patient. EFL frequently affects patients with COPD, obesity, and heart failure, as well as patients with ARDS, especially at low PEEP. EFL is, however, most often unrecognized in the clinical setting despite being associated with complications of mechanical ventilation and poor outcomes such as postoperative pulmonary complications, extubation failure, and possibly airway injury in ARDS. Therefore, prompt recognition might help the management of patients being mechanically ventilated who have EFL and could potentially influence outcome. EFL can be suspected by using different means, and this review summarizes the methods to specifically detect EFL during mechanical ventilation.

Les conséquences hémodynamiques de la ventilation mécanique sont multiples et complexes et peuvent affecter tous les déterminants de la performance cardiaque que sont la fréquence cardiaque, la précharge, la contractilité et la postcharge. Ces conséquences concernent tant le cœur droit que le cœur gauche, d’autant plus qu’il existe une interdépendance entre le ventricule droit et le ventricule gauche.

La mise sous ventilation modifie le volume pulmonaire mais aussi la pression intra-thoracique. Les variations du volume pulmonaire ont des conséquences sur les résistances vasculaires pulmonaires, la vasoconstriction pulmonaire hypoxique et l’interdépendance ventriculaire. Les variations de pression intra-thoracique ont un impact majeur et affectent le retour veineux systémique, la précharge ventriculaire droite, la précharge ventriculaire gauche, la postcharge ventriculaire gauche, la postcharge ventriculaire droite et la contractilité myocardique. Les conséquences hémodynamiques de la ventilation en pression positive vont dépendre des pathologies cardiorespiratoires chroniques sous-jacentes lors desquelles survienne l’insuffisance respiratoire aiguë posant l’indication de la mise sous ventilation.

Dans cette revue, nous décrirons en particulier la situation de la décompensation aiguë de BPCO, de l’insuffisance cardiaque gauche aiguë et du sevrage de la ventilation.

The haemodynamic consequences of ventilation are multiple and complex and may affect all the determinants of cardiac performance such as heart rate, preload, contractility and afterload. These consequences affect both right and left ventricle and are also related to the biventricular interdependence.

Ventilation modifies the lung volume and also the intrathoracic pressure. Variations in lung volume have consequences on the pulmonary vascular resistance, hypoxic pulmonary vasoconstriction and ventricular interdependence. Variations in intrathoracic pressure have a major impact and affect systemic venous return, right ventricular preload, left ventricular preload, right ventricular afterload, left ventricular afterload and myocardial contracility. The haemodynamic consequences of positive pressure ventilation depend on the underlying chronic cardiopulmonary pathologies leading to the acute respiratory failure that was the indication for ventilation.

In this review, we will focus on severe COPD exacerbation, acute left heart failure and weaning from ventilation.

To assess the safety of mechanical ventilation and effectiveness of extrinsic positive end-expiratory pressure (PEEP) (PEEP_e) in improving peripheral oxygen saturation (SpO₂) during direct microlaryngeal laser surgery; to assess the incidence, amount, and nature (dynamic hyperinflation or airflow obstruction) of ensuing intrinsic PEEP (PEEP_i); and to find a surrogate PEEP_i indicator.

Quasiexperimental.

S. Raffaele Hospital (Milano), November 2009 to December 2010.

Fifty-two adults scheduled for direct microlaryngeal laser surgery. Exclusion criterion is pregnancy.

Twenty-one percent O₂ mechanical ventilation through 4.5- to 5.5-mm internal diameter endotracheal tubes; in 29 patients, after measurement of PEEP_i, an identical amount of PEEP_e was added; and PEEP_i.

SpO₂, peak (Paw_peak) and plateau (Paw_plateau) airway pressure, and end-expiratory carbon dioxide were measured every 5 minutes. Respiratory compliance (C_rs) was computed. PEEP_i was measured (end-expiratory occlusion method).

PEEP_i ≥ 5 cm H₂O occurred in 14 patients (27%) after intubation, in 16 (30%) at the beginning, and in 14 (27.3%) at the end of surgery. Thirty-one patients (59.4%) exhibited PEEP_i ≥ 5 cm H₂O on at least 1 time point. PEEP_i at the beginning of surgery was positively correlated with Paw_plateau, C_rs, tidal volume, and body mass index. Body mass index was the only predictor for the occurrence of PEEP_i ≥ 5 cm H₂O. At the beginning of surgery, the Paw_plateau receiver operating characteristic curve predicting PEEP_i ≥ 5 cm H₂O had area under the receiver operating characteristic curve of 0.85; best cutoff value of 15.5 cm H₂O (sensitivity, 88.9%; specificity, 75%; correctly classified cases, 86.1%). When PEEP_e was applied, in 23 cases (82.1%), total PEEP equaled PEEP_e+ PEEP_i; in 3 (10.7%), it was lower; and in 2 (7.1%), it was higher. Application of PEEP_e increased SpO₂ (P< .05) and C_rs (P< .05).

During ventilation through small endotracheal tubes, PEEP_i (mostly due to dynamic hyperinflation) is common. Hemodynamic complications, barotrauma, and O₂ desaturation (reversible with PEEP_e) are rare. Paw_plateau provided by ventilators is useful in suspecting and monitoring the occurrence of PEEP_i and allows detection of lung overdistension as PEEP_e is applied.

Double-lumen tubes (DLT) are routinely used to enable one-lung-ventilation (OLV) during thoracic anaesthesia. The flow-dependent resistance of the DLT’s bronchial limb may be high as a result of its narrow inner diameter and length, and thus potentially contribute to an unintended increase in positive end-expiratory pressure (auto-PEEP). We therefore studied the impact of adult sized DLTs on the dynamic auto-PEEP during OLV.

In this prospective clinical study, dynamic auto-PEEP was determined in 72 patients undergoing thoracic surgery, with right- and left-sided DLTs of various sizes. During OLV, air trapping was provoked by increasing inspiration to expiration ratio from 1:2 to 2:1 (five steps). Based on measured flow rate, airway pressure (P_aw) and bronchial pressure (P_bronch), the pressure gradient across the DLT (ΔP_DLT) and the total auto-PEEP in the respiratory system (i.e. the lungs, the DLT and the ventilator circuit) were determined. Subsequently the DLT’s share in total auto-PEEP was calculated.

ΔP_DLT was 2.3 (0.7) cm H₂O over the entire breathing cycle. At the shortest expiratory time the mean total auto-PEEP was 2.9 (1.5) cm H₂O (range 0–5.9 cm H₂O). The DLT caused 27 to 31% of the total auto-PEEP. Size and side of the DLT’s bronchial limb did not impact auto-PEEP significantly.

Although the DLT contributes to the overall auto-PEEP, its contribution is small and independent of size and side of the DLT’s bronchial limb. The choice of DLT does not influence the risk of auto-PEEP during OLV to a clinically relevant extent.

DRKS00005648.